1. Field of the Invention
The present invention relates to an encoder used in, for example, a displacement sensor.
2. Description of the Related Art
An encoder generates a plurality of periodic signals different in phase from one another. If the periodic signals generated by the encoder are supplied to a processing circuit for counting cycles, the traveling direction, position, displacement and displacement velocity of an object under test can be measured.
FIG. 6 shows a configuration diagram of a conventional encoder. A resistance 102 for applying a predetermined current is connected in series to a light source 101. A light emitted from the light source 101 is applied to a scale 103. The scale 103 has a periodic optical pattern. The scale 103 moves relative to the light source 101. The light emitted from the light source 101 is applied to the scale 103 so that a reflected light, a diffracted light, a scattered light and a transmitted light are generated from the scale 103. The reflected light, diffracted light, scattered light and transmitted light enter two light receiving elements 104A, 104B. The light receiving elements 104A, 104B are disposed at corresponding positions different in phase 180 degrees from each other. The light receiving elements 104A, 104B output photocurrents corresponding to intensity changes of the reflected light, diffracted light, scattered light and transmitted light from the scale 103 in order to detect a movement amount of the scale 103.
The photocurrent generated by one light receiving element 104A is input to current/voltage converting means comprising a resistance 105A and an operational amplifier 106A and converted into a voltage signal VPA. The photocurrent generated by the other light receiving element 104B is input to current/voltage converting means comprising a resistance 105B and an operational amplifier 106B and converted into a voltage signal VPAB.
Each of the voltage signals VPA, VPAB contains a direct-current component which is a light component of a given intensity and noise equally included in both of the voltage signals VPA, VPAB, in addition to an alternating-current component which is a component of the intensity change from the scale 103.
Subtracting means 107 is connected to the subsequent stage of the current/voltage converting means in order to eliminate the noise. The subtracting means 107 performs an operation VREF-(VPAB-VPA) by use of a reference voltage VREF which appears due to a resistance 108 and by use of the voltage signals VPAB, VPA output from the current/voltage converting means, in order to eliminate the noise. As a result of the above operation, a periodic signal which only includes the intensity change from the scale 103 as a signal component is obtained. The reference voltage VREF is supplied to the operational amplifiers 106A, 106B and the subtracting means 107 and serves as a reference of the periodic signal.
The encoder performs the above-described signal processing, and obtains a plurality of periodic signals for use as encoder signals.
An encoder technique is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 6-26817.
The encoder has the processing circuit shown in FIG. 6 installed in an encoder head to process the photocurrents output from the light receiving elements 104A, 104B. The encoder also comprises an exclusive circuit for processing the photocurrents.
Some robots have hands equal in size to, for example, human hands. For example, there is a robot having a hand with five fingers. This robot has joints provided in the fingers of the hands. In order to provide the joints in the hands of the robot, about three encoders are needed for each finger of the hand. If a multijoint robot is configured, the number of joints in the hands of the robot increases. Accordingly, the number of encoders installed in the hands of the robot increases. To mount a great number of encoders in the multijoint robot, there are difficult aspects in the encoder size and in the degree of freedom of encoder attachment.
When the number of wiring lines for a great number of encoders is taken into consideration, at least a power supply V/GND and two signal lines are needed for the operation and detection of one encoder. Even if the power supply V/GND for all of the encoders can be supplied by the same wiring line, 30 or more signal lines are needed, for example, when encoders are installed for all the joints of five fingers. It is not easy to run these signal lines.